Since a device in which a micro-electromechanical system (MEMS) is applied to a variable capacitance element (which will be referred to as a MEMS variable capacitance device hereinafter) can realize low loss, high isolation and high linearity, it is expected as a key device that realizes multiband and multimode of next-generation mobile terminals.
The reason why the MEMS variable capacitance device can realize the low loss is that a metal material having a low resistivity is used for a capacitive electrode of the variable capacitance element which is a movable portion. Specifically, aluminum (Al) or gold (Au) is used for the capacitive electrode. Such a metal material generally shows ductibility characteristics.
Therefore, there is a problem that, when the movable capacitive electrode is repeatedly driven, creep (change in shape due to stress) of a member constituting the movable portion causes skew of the shape of the movable portion.
In the case of the MEMS variable capacitance device, when an electrode as a movable portion changes due to creep, an interval between two electrodes of a capacitive electrode changes from a predetermined interval. Therefore, when the MEMS variable capacitance device is used over a long period, a capacitance value of the variable capacitance element varies from a designed value.
Further, in a switch to which the MEMS is applied, isolation characteristics of a contact portion of the switch deteriorate.
As one of means for avoiding creep, a hard metal material that has small plastic deformation and shows brittleness characteristics like an aluminum-titanium (Al—Ti) alloy or tungsten (W) is used for the movable portion. However, such a metal material has a higher resistivity than Al or Au, and hence the loss increases.
Furthermore, as another means, a structure in which a metal material as a ductile material and an insulator as a brittle material are laminated is adopted for the movable portion (see, e.g., U.S. Pat. No. 7,299,538). However, in this laminated structure, the movable portion is apt to warp due to a difference in internal stress between the ductile material and the brittle material, and as a result, the deformation of the movable portion occurs.